1. Field of Invention
The present invention relates to an electricity supply element, in particular to an electricity supply element having a ceramic separator to enhance heat tolerance and the ion conductivity.
2. Related Art
The separator (film) is an important unit of an lithium battery. The separator is disposed between the positive electrode layer and the negative electrode layer. The separators are used to electrically insulate the negative and positive electrode layers so that the battery electrolyte forms the only ionically conducting path therebetween. In order for the separators to provide this insulating function, the separators must exhibit a low electrical resistance, must be chemically stable in the electrolyte environment, must resist stiffening and cracking, must be wettable to the battery electrolyte, and must limit active material transport.
The microporous polyolefin film, such as PE or PP, is widely used to manufacture battery separators because it is chemically stable and outstanding in physical properties. The melting point of the PE is about 130° C., and the melting point of the PP is about 160° C. Limited thermal and mechanical stability of the polymer separator may lead to overheating of the battery and explosion caused by separators melting and rupturing. Thermal operation window of separator in a battery is determined by shutdown temperature and melt down temperature. Therefore, in recent years, ceramic separators with high thermal stability and good wettability are developed.
There are two types of the ceramic separator. The first type is used the ceramic particles as the main material of the separator to replace the polyolefin, such as U.S. Pat. No. 5,342,709. The second type is utilized the ceramic particles to be coated on a film made of PET, PEN or PI, such as US patent application publication no. 2008/0138700.
For the first type, the ceramic particles of the separator are bonded with the electrode layer by the adhesive. The binder system of the adhesive is similar to the binder system of the electrode layers. Therefore, the solvent systems are also similar. When drying the solvent after coating, the particles rearrangement and the adhesive entanglement are occurred and the holes are produced in the bulk and at the interfaces. The holes would be a good path for ion mobility. However, due to the solvent is removed in a short time, the holes may be huge to induce micro-short. Therefore, the electrical insulation of the electrodes in battery will be compromised.
For the second type, the adhesive, such as Polyvinylidene fluoride (PVDF) or PVDF-HFP, is used to bind the ceramic particles on the surface of a substrate film. Moreover, the film has holes to permit the ion migration. The ceramic particles are coated on the substrate film meant to ensure the electrical insulation of the electrodes at abnormal conditions. However, the adhesion between the PVDF or PVDF-HFP, and material of the substrate film, such as PET, PEN or PI, is weak, and the ceramic particles are easy to peel off. To increase the particle's adhesion to the substrate film, the percentage of the adhesive has to be higher. However, the ion conductivity would be lowered accordingly.
Also, if the amount of the ceramic particles is raised to enhance the ion conductivity, the amount of the adhesive also has to be raised accordingly to maintain enough adhesion. Therefore, the percentage of the ceramic particles is limited, only 40% at most. The ion conductivity is limited. To increase the ion conductivity, the plasticizer or non-solvent are added into the adhesive. After the formation of the separator film, the plasticizer or non-solvent is removed to obtain holes for ion mobility to enhance the ion conductivity.
Furthermore, the ceramic particles may easily absorb water, which will significantly deteriorate battery performance. To remove the absorbed water of the ceramic particles, it has to be heated at a temperature over 190° C. However, the melting point of the adhesive is not high enough. For example, the melting point of the PVDF is about 170° C., and the melting point of the PVDF-HEP is about 120-150° C. When the separator is heated at 190° C., the adhesive would melt. The distribution of the holes within the separator would be changed to worsen the electric property of the battery. Even adding the plasticizer or non-solvent as above mentioned, the holes would be refilled by the molten adhesive to lower the ion conductivity.
On the other hand, if the materials of the adhesive with high heat tolerance are used, such as epoxy resin, acrylic acid resin, polyacrylonitrile (PAN), the thermal stability and the adhesion will be improved. However, these materials would form such cross-linking network with dense structure which significantly reduces ion migration and final battery performance. In other words, the distribution of the holes is not suitable for ion migration. Therefore, the ion conductivity is very low.